In known types of nuclear power reactors, for example as used in the Dresden Nuclear Power Station near Chicago, Ill., the reactor core comprises a plurality of spaced fuel assemblies arranged in an array capable of self-sustained nuclear fission reaction. The core is contained in a pressure vessel wherein it is submerged in a working fluid, such as light water, which serves both as coolant and as a neutron moderator. Each fuel assembly comprises a removable tubular flow channel, typically of approximately square cross section, surrounding an array of elongated, cladded fuel elements or rods containing suitable fuel material, such as uranium or plutonium oxide, supported between upper and lower tie plates. The fuel assemblies are supported in spaced array in the pressure vessel between an upper core grid and a lower core support. The lower tie plate of each fuel assembly is formed with a nose piece which fits in a support socket for communication with a pressurized coolant supply chamber. The nose piece is formed with openings through which the pressurized coolant flows upward through the fuel assembly flow channels to remove heat from the fuel elements. A typical fuel assembly of this type is shown, for example, by B. A. Smith et al in U.S. Pat. No. 3,689,358. An example of a fuel element or rod is shown in U.S. Pat. No. 3,378,458.
Additional information on nuclear power reactors may be found, for example, in "Nuclear Power Engineering," M. M. El-Wakil, McGraw-Hill Book Company, Inc., 1962.
A typical fuel assembly is formed, for example, by an 8.times.8 array of spaced fuel rods supported between upper and lower tie plates, the rods being several feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the fuel rods in fixed spaced relation and restrain them from bowing and vibrating during reactor operation. A plurality of fuel rod spacers positioned in spaced relation along the length of the fuel assembly are provided for this purpose. Such spacers are shown, for example, by J. L. Lass et al. in U.S. Pat. No. 3,654,077.
A problem in the design of such a fuel assembly is to provide an efficient, effective structure for maintaining the fuel rod spacers in their axial spaced positions without the use of excessive structural materials. In some previous arrangements, special structural members have been provided for this purpose. It is extremely important to minimize the amount of structural material in a fuel core because such materials unproductively capture neutrons and an additional amount of costly fuel is required in the core to compensate for this neutron loss. Thus it is undesirable to use a structural member whose only purpose is to retain the spacers. In other known arrangements the spacers are axially retained by engagement with lugs or the like on one or more of the fueled rods. However, use of a fueled rod for spacer retention presents problems of high temperature strength in the face of the need to minimize cladding thickness in a fueled rod.
An efficient and effective arrangement for axially retaining the spacers is shown by J. R. Fritz et al. in U.S. Pat. No. 3,802,995 (which is incorporated herein by reference) wherein the spacers are retained in axial position by engagement with lugs projecting from a coolant conducting tube located in an interior position in the fuel assembly, this coolant conducting tube providing the additional function of increasing the neutron moderation in the inner zone of the fuel assembly.
As the trend toward higher fuel burnup exposure (i.e. longer residence time of the fuel assemblies in the core) continues a potential problem attendant the use of the spacer retaining coolant conducting tube arises. The problem is the possibility that the upper or lower end plug shank of the coolant conducting tube (hereinafter water tube) may become disengaged from its support cavity because of the greater axial growth (elongation) of the fuel rods in the reactor core environment as compared to the axial growth of the water tube.
In the high-temperature, high-neutron and gamma radiation environment of the reactor core both the fuel rods and the water tubes experience permanent axial growth. However, the axial growth of the fuel rods is significantly greater because of mechanical interaction of the fuel element cladding tube with the fuel pellets therein and the higher temperatures experienced by the cladding tube metal. The greater the exposure the greater the axial growth and, more to the point, the greater the differential growth between the fuel elements and the water tube. If this differential growth becomes great enough the previously mentioned potential problem of water tube end plug shank disengagement arises. Should such disengagement occur, coolant flow induced vibration of the now unsupported end of the water tube might become of sufficient magnitude to result in water tube contact with and potential damage to the adjacent fuel elements.
An object of the invention is an improved water tube-fuel rod spacer retaining arrangement.
A more specific object is a water tube arrangement which substantially precludes water tube disengagement and minimizes coolant flow induced vibration of the water tube.